Inverters have recently become available that utilize power darlington transistors having the capability to switch 20 amperes of current with control currents of 40 milliamperes. These transistors are commercially available in complementary pairs consisting of one pnp and one npn type. Circuits for controlling these transistors typically require both level shifting and amplification of control currents to levels sufficient to effect transistor switching. The most common devices used for this purpose are drive transformers because they can increase drive currents by the ratio of primary to secondary turns while simultaneously isolating the control circuits from the load. Major disadvantages that have been found with the use of transformers are their size, weight, electromagnetic interference (EMI) shielding requirements and high cost. This invention eliminates the above disadvantages by disclosing unique drive circuitry which does not require the use of transformers or any other load decoupling devices.
In accordance with the above, the invention discloses a method and an apparatus having at least one inverter for converting a d.c. voltage to an alternating voltage for application across a load, the inverter comprising a first transistor, a second transistor having its collector connected to the first transistor collector and to the load input, a d.c. power supply having an upper supply voltage output and a lower supply voltage output, the upper voltage output being connected to the first transistor emitter and the lower voltage output connected to the second transistor emitter, a means for alternately applying the upper voltage and lower voltage at the first and second transistor emitters, respectively, to the collector connection point, and a means for preventing simultaneous conduction of the first and second transistors.
In an embodiment in which the load return is reference to a voltage level between the upper and lower supply voltage output levels of the d.c. power supply, the first transistor will be a pnp type and have its emitter connected to the upper supply voltage and the second transistor will be an npn type and have its emitter connected to the lower supply voltage. The invention also discloses circuitry in which the first and second transistors are controlled through their respective base terminals by the upper and lower supply voltages and upper and lower reference voltages which are proportional to the upper and lower supply voltages.
Also disclosed is an apparatus for powering a three phase synchronous motor in which one inverter is used for each of the three motor windings. In this apparatus synchronism of the three inverters with respect to each other is maintained by the relative phasing of three input square wave clock pluse signals, each of which is phase-separated by 120 degrees from the other two.